The attenuation of transmitted x-rays incident upon and emergent from an object, in particular as attributable to photoelectric absorption, has been used as the basis for screening objects to obtain some form of representational data representative of the contents or components thereof relative to each other, and for example to generate an image.
The data can give information at various levels. Even if intensity data is collected monochromatically across a broad spectrum, attenuation is known to vary both with the thickness/density of an object. By use of suitable detectors and a suitable source, radiographs of an item under test in the form of images based on the absorption behaviour of an object or its contents or components can be generated.
This method tends to give limited information about the material content. In essence, at its simplest, all that is being measured is transmissivity of the object to the source radiation. The detector merely collects amplitude information, and does not discriminate transmitted radiation spectroscopically.
However, it is known that spectroscopic information from transmitted x-rays could be used to give additional information about the material content of the objects or components being scanned. It is known that the x-ray absorption properties of any material can vary spectroscopically with incident x-ray photon frequency/energy, and that this effect depends in particular on atomic number. This has led to development of dual-band or dual-energy detectors which are capable of separately identifying low- and high-energy bands from the spectrum of x-ray emissions of a suitable source. Such detectors are described for example in U.S. Pat. No. 4,626,688.
A dual-energy system thus confers some limited information about composition. For example, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category, and a or dual-energy detector can thus make an approximate organic/inorganic discrimination. However the organic/inorganic division is crude and approximate. Conventional dual-energy detectors give limited real spectroscopic information about the spectrum of transmitted x-rays.
Multiple sources and/or filters and/or detector arrays can be used to produce a system with the ability to resolve into further bands across a source spectrum even if individual detectors are inherently monochromatic or dual-energy.
Recent development of detectors that can resolve spectroscopic information about the transmitted x-rays more effectively has led to the development of apparatus that inherently simultaneously discriminates across a larger range plurality of energy bands, for example to generate multispectral images. For example U.S. Pat. No. 5,943,388 describes a system that makes use of the inherent ability of cadmium telluride detectors to resolve incident x-rays spectroscopically across a broad spectrum to produce intensity data across at least three energy bands simultaneously and generate at least three images. This better exploits the effect of differential spectral absorption by different materials and better approximates transmissivity to composition but is still limited to the information that can be conveyed by a displayed image, and by the approximate and indicative nature of any relationship between colour on a multispectral image, especially based on relatively wide energy bands, and composition of material in the transmission path.
If it is additionally desirable to generate specific compositional information from a spectroscopically resolved intensity dataset, and in particular to supplement that available by plural band imaging alone, the intensity data may further be numerically processed to fit the measured spectroscopically resolved intensity to known relationships for x-ray attenuation, for example with reference to the incident spectrum. An example of such an analysis is described in WO2008/142446.
Such techniques can be very powerful, particularly in applications where it is desired to gain accurate materials characterisation from objects comprising multiple component materials, such as airline luggage, in order to identify with precision particular to compositions of matter, for example comprising high molecular weight organics, which may be indicative of the presence of contraband materials in the luggage. However, because of the fine compositional distinctions which it is necessary to make in such applications, and the complexity of calculations that may therefore be involved in fitting intensity data to appropriate numerical relationships, the method can be relatively time consuming and can test the resolution limit of the detector material.
An alternative approach, which does not involved such degree of numerical analysis, might be preferable where it is desirable to develop a detector device, apparatus and method for the inspection and characterisation of materials in other situations, for example where such complex and close material distinctions need not be made and/or where a high throughput rate is desired.